Most radio transmitting antennas use electrons in a conductor to generate their radiated fields. But any charged particle, when accelerated, radiates electromagnetic energy. Electrons are generally easy to use because they are readily contained in wires and they can be accelerated with electric potentials. Typically, an electric oscillator is connected to the antenna to put a sinusoidally varying (or other appropriate) voltage on the wire. This potential causes the electrons to be accelerated at the frequency of the oscillator and produces a carrier frequency which is radiated. Modulating the frequency, amplitude, or phase of the carrier adds the communications information to the signal. Dominating the design of wire antennas in the kilohertz range and below is the limited number of electrons that can be put in a wire before resistive heating melts the wire. The currents and arrays of wires required to produce effective fields at such frequencies are very large.
Antennas may also use ions as the charge carriers. Singly charged ions carry the same charge magnitude as an electron and may be given the same acceleration as electrons to cause them to radiate exactly the same electromagnetic energy as electrons. A body of ions can be modulated effectively with electric or magnetic means to produce the same modulated carrier as from the electric wire antenna. The strength of electric and magnetic fields required to modulate ions is greater than that required to modulate electrons because ions have more mass. The typical ion antenna uses a resonance of a molecular ion to generate microwave frequencies.
The present invention uses hydrodynamic acceleration of the ions. It does so by entraining the ions in a neutral gas and inducing a desired acoustic motion through the gas. The ions, by being the same size and having a mass similar to the surrounding gas molecules are carried by the neutral gas which imposes its bulk motion on the ions. The acoustic field in the gas is a propagating sequence of waves of acceleration. The acoustic accelerations of the gas also accelerate the ions and cause them to radiate in proportion to the acceleration. This mechanism is suitable for generating frequencies up to that determined by the speed of sound in the chamber.
A significant benefit of the plasma antenna is that a greater number of ions can be generated in a plasma volume than the number of electrons that can be induced in the skin of a wire. In addition, a compact plasma container can take advantage of the major benefit of vertical orientation in launching radiation into the earth-ionosphere waveguide. Ion density and vertical orientation allow a plasma antenna with a volume of a few cubic meters to rival the performance of existing electronic antennas that are tens of kilometers in length for the lowest frequencies. The ion antenna also eliminates the problems of the ground connections of ground-loop antennas, and by reducing the local electric field, eliminates the need for environmental monitoring and the impact on local utilities.
The mechanism of the present invention overcomes the problems of wire antennas operating in the Extremely Low Frequency (ELF) (30 to 300 Hz) range, for example. Each of the existing ELF transmitter antennas has a dipole moment of 6.6.times.10.sup.6 ampere-meters. Approximately 10 cubic meters of a plasma with an ion density of 10.sup.20 ions per cubic meter to launch an equally effective electromagnetic wave into the atmosphere. Such densities of ions are produced routinely in experimental magneto-hydrodynamic generator flames that are seeded with materials of low ionization potential. The heat of combustion provides enough molecular energy to strip an outer electron from neutral atoms. An additional example of a mechanism to generate ions is with an arc discharge.
The amount of energy needed for ionization (and the resulting ionization fraction for a given temperature) depends on the atom species used for the ions. Cesium, rubidium, sodium, and potassium have low ionization potentials and can yield ionization fractions on the order of 10.sup.-4. Using such seed materials gives the flame an ion density in the range required for the acousto-ionic transmitter. The seeded flame produces the charge carriers that must be accelerated to make a carrier frequency which is, in turn, modulated to support communications.
Accelerating the ions in the stream can be accomplished by physically moving them by any number of mechanical means such as having the combustion chamber resonate in the manner of an organ pipe at the desired carrier frequency. For electromagnetic signal propagation to occur, the ions must be in the open air or in a container that is relatively transparent to electromagnetic waves. An organic-matrix composite material reinforced with non-conductive fibers such as glass can be used if it is internally insulated or actively cooled to protect it from the heat of the combustion gas. Small chambers with relatively high frequencies may be made with monolithic materials such as fused quartz that are resistant to the hot gas. In order to obtain the greatest acceleration of ions, the resonant frequency of the pipe is made to coincide with the natural frequency of the gas body in the pipe. The chamber is mounted such that it is rigidly affixed to a massive foundation so as to allow the chamber to flex in the desired mode of vibration. For example, if the longitudinal mode of vibration is desired, then the chamber is mounted with one end firmly anchored and the other end free to move longitudinally. A sliding anchor or an attachment with a two-pinned link allows the required motion. If radial motion is desired, then the main anchor is placed on one side of the chamber so as to allow the chamber to bulge radially at all other locations on its circumference. A chamber designed to have both longitudinal and radial frequencies coincident with that of the gas body provides optimum signal generation.
A shape superior to the organ-pipe uses the acoustic reflectivity of the chamber's walls to concentrate acoustic waves to a focus. The high intensity of acoustic pressure is accompanied by large molecular accelerations and usable radiation. The shape is essentially that of an ellipsoid. The end bells reflect acoustic energy to a large intensity at the foci of the ellipse. The acoustic energy to overcome losses is supplied by an attached horn that applies input energy in step with the resonant wave motion in the ellipse. The ions at the foci experience large accelerations and radiate electromagnetic power according to Larmor's formula. ##EQU1##
Where
q=charge PA1 acc=acceleration PA1 .delta..sub.o =permittivity of free space PA1 c=speed of light
The frequency of the acoustic tone of the organ-pipe chamber and the resulting frequency of the radiated carrier can be modulated by changing the resonant frequency of the pipe. This is accomplished by changing the length of the pipe, the impedance at the exit, or the stiffness of the pipe's wall. The longitudinal resonance of the chamber is proportional to its length, diameter, and wall thickness. The length may be changed by having more than one set of anchors near one end. The lowest frequency of the chamber is excited when the anchor that offers the longest free distance between the anchor and the free end is available. A higher frequency is generated when an anchor that is intermediate in distance restrains the chamber. Mechanical actuation of anchor position, that is, coupling and decoupling the intermediate anchor, causes the resonant frequency of the chamber to be modulated between the lower and the higher frequencies. This technique is the same as the finger restraint on the strings of a guitar at the frets on its fingerboard.
Modulation of the radial frequency may be accomplished with radial anchors, as in an analogy to the case of the longitudinal frequency, or the stiffness of the wall may be changed with circumferential bands that are coupled or decoupled, as needed. When the bands are not snug on the chamber, the frequency of the chamber is low. Actuating the bands to grip the chamber adds the stiffness of the bands to that of the wall of the chamber and increases the frequency. Actuators may be electro-mechanically operated. A more direct and rapid action results if a piezo-electric actuator is used.
Changing the resonant frequency of the gas may be accomplished by adjusting the geometry of the exit path from the gas chamber. The restricted opening acts as a nozzle and establishes the impedance mismatch between the inside of the pipe and the open air that induces longitudinal reflections, which excite all the other modes of vibration of the system. Changing the diameter of the throat changes its impedance and the resulting excitation frequency. The geometry of the throat may be changed by making it sufficiently flexible to have its generally round shape deflected to an oval. The impedance of the oval is different enough from that of a circle to change the resonant frequency of the chamber. A potentially simpler mechanism can impose a small flapper in the throat. However, such a mechanism would be suitable only for a relatively slow change in frequency.
A modulation scheme that produces a shift of phase in the transmitted signal requires the use of the closed, ellipsoid-shaped, resonant chamber to allow the reflective properties of the walls to be manipulated. It is the walls that impose the phase shift on the driving gas and the driven ions.
The implementation of a modulator that shifts the phase of the fixed-frequency carrier requires a reflective surface that can be changed quickly. A normal reflection from a large mismatch of acoustic impedance causes a 180-degree phase shift in an incident wave. Moving the wall toward or away from the incident wave at a speed faster than the speed of sound in the chamber causes an apparent phase shift of the reflection. Mechanically actuating the position of a barrier so quickly is difficult, although an alternative is available. It consists of a surface with an adjustable acoustic impedance. The surface is a reflector body made of an electro-rheological fluid, a material that changes its bulk modulus, and thereby its acoustic impedance, with the application of an electric field.